In oil and gas fracturing operations, produced water contributes a significant amount of wastewater. According to American Petroleum Institute (API) statistics, for each barrel (bbl) of oil produced, an average of 7.5-10 bbl of water will be produced. The produced water contains dispersed oil and grease, produced solids, organic and inorganic chemicals, salts, bacteria, metals, and sulfate, etc. The contaminants exist in produced water in a variety of physical states including solution, suspension, emulsion, adsorbed particles, and particulates. The most common method for produced water disposal includes separating the oils and suspended solids by flotation and gravity settling and injecting the treated water into deep-wells, as permitted.
Dissolved air flotation (DAF) is a process whereby micro-air-bubbles attach themselves to suspended materials causing them to float to the surface of a flotation chamber to achieve liquid/solids separation. DAF systems are frequently used to provide wastewater pretreatment, product recovery, and capture and thicken biological solids in industries ranging from food processing to pulp and paper as well as to oil and gas, and petrochemical industries.
The modern DAF technology utilizes the air dissolving pump to generate the micro bubbles required to float the solid particles in wastewater. Flow enters the influent chamber with air saturated water provided by the air dissolving pump. Air is introduced into the pump suction and the bubbles are sheared into much smaller bubbles by the pump and then dissolved into the water by high pressure. A polymer, or other coagulants, will be added into the influent mixing and coagulation chamber to enhance the removals of solids or oil.
The water then enters the flotation chamber by passing over the influent chamber dividing wall. The velocity of the water in the flotation chamber is significantly reduced to maximize separation potential. Inside the flotation chamber, the micro-bubbles (saturated water mixture), which have attached themselves to the particle's surface, change the particle's density. This causes the previously suspended solids to float to the surface where paddle assemblies skim them from the surface into a sludge trough.
Heavy grit and solid particles settle onto the bottom of the DAF where they are flushed out into the sludge system via a manual valve. The clarified liquid then enters the effluent chamber and passes over an effluent weir into an effluent box.
However, the conventional DAF technology itself can only remove some of the contaminants in the produced water, such as oil and grease, suspended solids, as well as a certain amount of the organics. A multifunction treatment system is eagerly demanded by the oil and gas industry in order to fulfill the produced water discharge limits and to protect the injection well and/or to extend injection well life time.
The present invention described below addresses the foregoing problems, combining the technologies of oil recovery, solids removal, emulsion breaking, and advanced oxidation together and gives the oil and gas industry a compact, mobile, simple operation, and multifunction solution. In addition to the oil and gas industries, this invention can also be applied in food processing, pharmaceutical, agricultural, as well as municipal wastewater pre-treatments.